1. Field of the Invention
The present invention is broadly concerned with improved microwire temperature sensors which are specially designed to be partially or wholly embedded within a heat treatable or curable material, such as Carbon Fiber Reinforced Plastic (CFRP), other curable polymers, elastomers, rubber compounds, ceramic materials, cement, or concrete, in order to accurately sense the temperature of the material during the course of heat treatment (e.g., a pre- and/or post-curing temperature range). More particularly, the invention is concerned with such sensors which have at least one primary temperature-sensing microwire housed and encased within a substantially rigid, close-ended protective tube formed of a material which does not appreciably magnetically bias the re-magnetization responses of the microwire(s); in preferred forms, a plurality of microwires, including the primary temperature-sensing microwire and/or a calibration microwire and/or a reference microwire are positioned within the protective tube. The use of protective tubes isolates the microwires from stresses developed in the material during heating or curing thereof to provide more accurate temperature sensing. Furthermore, these protective tubes prevent the microwires from strain caused by the material's final post-cure dimensions being different than starting dimensions (for instance, due to shrinkage), where such strain can alter the re-magnetization responses of the microwire at any given temperature and thus cause inaccurate temperature measurements. Finally, these protective tubes isolate the microwires from stress during post-cure heating or cooling of the material due to expansions and contractions, thus allowing accurate temperature measurement of the material or part at any time post-cure.
2. Description of the Prior Art
U.S. Patent Publications Nos. 2007/0263699 entitled “Magnetic Element Temperature Sensors,” 2008/0175753 entitled “Microwire-Controlled Autoclave and Method,” and 2010/0322283 entitled “Magnetic Element Temperature Sensors” describe the general construction and operation of microwire temperature sensors and overall temperature-sensing apparatus used with such sensors. Broadly speaking, these microwire sensors include at least one temperature-sensing microwire comprising an amorphous or nanocrystalline metal alloy core in the form of an elongated wire or ribbon, with a glass sheath or coating surrounding the core (see FIG. 1 of the '753 publication). The metal alloy cores of such temperature-sensing microwires are carefully designed so that the microwires exhibit characteristic re-magnetization responses under the influence of an applied alternating magnetic field, and moreover have set point temperatures (usually the Curie temperature or any number of temperatures in a region below the Curie temperature) such that the re-magnetization responses of the microwires are significantly different below or above these temperatures. The re-magnetization responses are defined by at least one short, detectable pulse of magnetic field perturbation of defined duration. Once Curie temperature of the microwire is reached, the detected voltage of the response goes to zero or near-zero.
These prior microwire sensors are used in conjunction with detecting apparatus including a field-generating coil which generates an interrogating alternating magnetic field, and a detection coil, which detects the re-magnetization responses of the interrogated microwires. These responses are then decoded using a signal processor to generate an output representing the temperature experienced by the sensors. Hence, the temperature experienced by a heat curable material having a microwire sensor embedded therein can be monitored. Furthermore, this temperature information can be used to control the heating process.
While the utility of known microwire temperature sensors and related detection apparatus has been established, certain problems have been encountered when the prior art sensors have been permanently embedded within uncured, layered CFRP composite materials, such as those used in the fabrication of aerospace parts, followed by heat treatment in an oven or autoclave to cure the composite. The goal was to remotely and wirelessly detect the temperature of the interior of the composite object during the curing process, so that the temperature information could be used by a controller to adjust the oven/autoclave temperatures and/or elapsed curing time at the various temperatures, to yield a more effective and efficient curing cycle.
However, in actual practice it was found that the embedded prior art sensors did not provide completely reliable temperature sensing, when comparatively tested using embedded Type E thermocouples. The sensed temperature profile from the prior art sensors was, in certain instances, significantly different from that derived using the thermocouples.
There is accordingly a need in the art for improved microwire sensors which can be at least in part embedded within heat-curable materials, such as CFRP composites or other materials to give accurate temperature data during the heat curing process or exotherm process, as well as during service thereafter.